The typical American baby born in 1979 could expect to live about 73.9 years, while the typical baby born in one of the other 34 OECD countries would live roughly to age 72.3.

But by 2015 that gap had flipped. The average American born that year could expect to live a little less than 79 years, while the typical baby born in an OECD country had an expected life span of nearly 81 years.

In 2016, U.S. life expectancy dropped for the second year in a row, a statistical event that hasn't happened since the early 1960s. Numbers for the remaining OECD countries aren't yet available, but if prior trends continue, the gap between the United States and the rest of the wealthy world is likely to grow even larger.

The United States remains one of the wealthiest countries in the world. So what happened?

We can start with our health-care system, which is frankly something of a mess. We spend thousands of dollars more per capita on health care than any other country in the world, but in return we live shorter lives than people in most other rich nations.

Violence is also taking a toll on our life expectancy. While our homicide rate has been steadily falling since the early 1990s, Americans are still more likely to be murdered than people in nearly any other rich nations. A 2016 study found that “US homicide rates were 7.0 times higher than in other high-income countries, driven by a gun homicide rate that was 25.2 times higher.” Easy access to guns is the big factor there.

The United States also stands out for the stinginess of our social safety net relative to other rich countries. A 2014 review noted while plenty of individual factors lurk behind our short life spans — tobacco use, obesity, violence and disease among them — the lion's share of the difference between American life spans and those in other countries can be explained by “variations in non-medical determinants of health, some of which result from dramatic differences in public policies across the US and other OECD countries.”

Because the American tax code is more generous to the wealthy than tax systems in other rich countries, U.S. income inequality is among the highest in the OECD.

A study published in December of last year found that if these and other social welfare factors were brought up to the OECD average, it would add nearly four years to our collective life expectancy. “The US mortality disadvantage is, in part, a welfare state disadvantage,” the authors concluded.